add invalidate_inactive_shared_connections test function

We introduce invalidate_inactive_shared_connections udf to be used in
testing. It is possible that a connection count for an inactive node
will be greater than 0 and in that case it will not be removed at the
time of invalidation. However, later we don't have a mechanism to remove
it, which means that it will stay in the hash. For this not to cause a
problem, we use this udf in testing.

src/backend/distributed/connection/shared_connection_stats.c

@@ -110,7 +110,6 @@ static uint32 SharedConnectionHashHash(const void *key, Size keysize);
 
 PG_FUNCTION_INFO_V1(citus_remote_connection_stats);
 
-
 /*
  * citus_remote_connection_stats returns all the avaliable information about all
  * the remote connections (a.k.a., connections to remote nodes).

src/test/regress/expected/ensure_no_shared_connection_leak.out

@@ -117,6 +117,13 @@ SELECT pg_reload_conf();
 
 \c - - - :master_port
 SET search_path TO ensure_no_shared_connection_leak;
+-- invalidate inactive shared connections
+SELECT invalidate_inactive_shared_connections();
+ invalidate_inactive_shared_connections
+---------------------------------------------------------------------
+
+(1 row)
+
 -- ensure that we only have at most citus.max_cached_conns_per_worker
 -- connections per node
 select

src/test/regress/sql/ensure_no_shared_connection_leak.sql

@@ -54,6 +54,9 @@ SELECT pg_reload_conf();
 \c - - - :master_port
 SET search_path TO ensure_no_shared_connection_leak;
 
+-- invalidate inactive shared connections
+SELECT invalidate_inactive_shared_connections();
+
 -- ensure that we only have at most citus.max_cached_conns_per_worker
 -- connections per node
 select